In a conventional semiconductor integrated circuit formed on a semiconductor substrate, a plurality of metal wirings are formed by patterning a metal film such as an aluminum alloy film in order to connect semiconductor elements on the substrate to each other or to a bonding pad. In a multilayer wiring structure, the metal wirings on the outermost surface are coated with a moisture-proof insulating protective film for preventing corrosion of the metal wirings, then an aperture is formed to expose the bonding pad, and thus the wiring process is completed.
FIGS. 7A and 7B are cross-sectional views showing a final step of a typical wiring formation in a conventional manner. These figures show wirings arranged at a proper spacing within an integrated circuit, together with a bonding pad. As shown in FIG. 7A, an insulating film 2 is formed on a semiconductor substrate 1. The insulating film 2 comprises in general a multilayer film comprising, for example, a silicon oxide film of BPSG (boro-phospho-silicate glass) and TEOS (tetraethylorthosilicate). Though not shown in FIG. 7A or 7B, active elements such as a transistor are formed on the semiconductor substrate 1 and covered with the insulating film 2. Though not shown in FIG. 7A or 7B, the elements including the transistor pass through the insulating film 2 so as to be connected to internal wirings. A circuit internal wiring 3 comprising an aluminum alloy film and a bonding pad 4 are formed on the insulating film 2, further on which a protective film 5, such as a PSG (phospho-silicate glass)/silicon nitride (SiN) film and a SiN film, is formed. Furthermore, a resist film 6 is applied thereto. Next, as shown in FIG. 7B, the resist film 6 is used as a mask for etching the protective film 5. An aperture 7 for wire bonding is formed on the bonding pad 4 so as to expose the surface of the bonding pad 4, and then the resist film 6 is removed.
JP 61 (1986)-59739 suggests a method of laminating a silicon nitride film and a silicon oxide film on an aluminum wiring and forming an electrode takeoff by etching with a use of a resist as a mask.
In a large-scale semiconductor integrated circuit, several dozens of wirings are arranged in parallel and the wirings are often bent at a right angle. Such wirings are formed, e.g., in the vicinity of a power source around a semiconductor chip or around a memory circuit part. FIG. 8 shows corners of three aluminum alloy wirings 8. As shown in FIG. 8, the wirings can cause defects 9 in the insulating protective film in clearances between the wirings at the corner. The defects 9 are found during an examination of the protective film surface from which the resist film has been removed subsequent to formation of an aperture for wire bonding in accordance with the step indicated in FIGS. 7A and 7B.
The defects in an insulating protective film may allow water or the like to penetrate into the wiring layer disposed beneath and cause corrosion. As a result, reliability as a semiconductor integrated circuit will be damaged considerably.